I built my duckhouse 8'W X 4'D X 40"H with a 6'W X 10'L X 40"H wire pen. 2 windows for light in the front, inside the wire pen to keep predators out and let air in, a door that doubles as a ramp on one side for easy access for the ducks and can close it at night at bedtime (extra security from predators, also inside the wire pen) and a door on the other side (outside the wire pen) for MY access for cleanout of soiled straw and collection of eggs, when they start laying. I built 3 nesting boxes inside the duckhouse, where they will lay their eggs, and placed them just inside of the cleanout door.
I use straw on the inside floor because it is easier to remove at cleanout time. You can also use pine wood shavings. Don't use hay as hay will mildew and mold. Also, never use cedar shavings as cedar is toxic to ducks and chickens if they eat it, which they will.
I built an access door on the wire pen, right at the door/ramp on the house, so I can close the door/ramp at night, and another one the full width of the wire pen at the end of the pen so I can pull out the kiddie pool to empty it and wash it out, then slide the pool back in and refill it with clean water.
I have 1, 10 week old mallard, 4, 8 week old pekins, 3, 3 week old mallards and 1, 3 week old sweedish/muscovy mix, (the 3 week olds I hatched out and they are not old enough to go in the pen with the older ones yet).
My duckhouse should be big enough for all 9 of my ducks. The wire pen may have to be extended when the 4 little ones are big enough to be put with the others.
I will try to take some pix tomorrow and post them for your reference.
Good luck!! Ducks are some messy creatures....lol.
Tim